feat(infra): Objecten publishes register events to NRC (refs #152)

Builds the four pieces ADR-0028 deliberately left absent, and turns
`NOTIFICATIONS_DISABLED` back off:

- `objecten-celery`, a worker on the Objecten image (mirrors `oz-celery`), plus
  `CELERY_BROKER_URL`/`RESULT_BACKEND` on objecten-redis db 1 (db 0 is the cache).
  Without it `notifications_api_common` queues the send and nothing ever ships it.
- An `nrc` service + `notifications_config` in Objecten's setup_configuration,
  reusing the `big-reference-seed` credential OpenZaak publishes with.
- The `objecten` kanaal in NRC's setup_configuration — the name is fixed by the
  Objects API (`NOTIFICATIONS_KANAAL`), and publishing to an unregistered kanaal is
  what the failing check reported first.
- `SITE_DOMAIN: objecten.local:8000` + an `objecten.local` network alias: NRC
  validates `hoofdObject`/`resourceUrl` with Django's URLValidator, which rejects a
  single-label host, so `objecten:8000` is refused with "Voer een geldige URL in."
  The alias keeps the dotted host resolvable so the URL still dereferences.

ADR-0029 records it; ADR-0028's ceiling now points there.

Makes `make verify-objecten-notifications` (dc9ca2c) pass.

docs/architecture/adr-0028-objecten-holds-the-register.md

@@ -119,8 +119,8 @@ every message was dropped on the floor — a delivery path that looks wired and
 
 - ponytail ceiling: Objecten emits no notifications, so nothing downstream can react to a
   register write yet.
-- Upgrade path: S-19b (#150) needs those notifications to source the projection from
-  Objecten, and turns them on together with the broker, worker, kanaal and abonnement.
+- **Lifted by ADR-0029** (S-19b-1, #152): broker, worker, `objecten` kanaal and
+  notifications config now exist, and `NOTIFICATIONS_DISABLED` is `false`.
 
 ## Consequences